Circuit arrangement for producing pedestal current in a pyroelectric thermo imaging tube

ABSTRACT

A pyroelectric thermo imaging tube employing an external circuit arrangement for the variable pulsing of the cathode during horizontal flyback periods that are not coincidental with vertical flyback periods to remove residual charges as well as opposite polarity charges and potentials generated on the nonconducting side of the pyroelectric target. Pulsing of the cathode provides secondary electron emission which produces a pedestal current that can be employed for the direct read-out from the target.

United States Patent Crowell 1111 3,812,396 [451 May 21, 1974 PrimaryExaminer-Leland A. Sebastian Assistant Examiner- P. A. Nelson Attorney,Agent, or Firm-Frank R1 Trifari [5 7] ABSTRACT A pyroelectric thermoimaging tube employing an external circuit arrangement for the variablepulsing of the cathode duringhorizontal flyback periods that are notcoincidental with vertical flyback periods to remove residual charges aswell as opposite polarity charges and potentials generated on thenonconducting side of the pyroelectric target. Pulsing of the cathodeprovides secondary electron emission which produces a pedestal currentthat can be employed for the direct read-out from the target.

8 Claims, 5 Drawing Figures [75] Inventor: Merton I-I. Crowell, Mahopac,N.Y. [73] Assignee: North American Philips Corp., New

York, N.Y. 221 Filed: Dec. 11, 1972 [2]] App]. No.: 313,832

[52] US. Cl. 315/10, 250/332 [51] Int. Cl. H0lj 31/26 [58] Field ofSearch 315/10, 11, 12; 250/330, 250/332, 333; 252/629 [56] ReferencesCited UNITED STATES PATENTS 3,056,062 9/1962 Garbuny et al. 3l5/ll3,089,055 5/1962 Lehrer 315/12 3,144,578 8/1964 Sinclair 315/12.

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l9 2o 4 l l M I l M v \vvw l U M U I "h HI n "hi8 CIRCUIT ARRANGEMENTFOR PRODUCING PEDESTAL CURRENT IN A PYROELECTRIC THERMO IMAGING TUBE IBACKGROUND OF ru s INVENTION The invention relates to pyroelectricthermo imaging camera tubes, and more particularly to methods of readingout extremely small variations of potentials and charges onnon-conducting electrodes.

Pyroelectric thermo imaging tubes are well-known, such as disclosed incopending patent application (154-10-911), filed concurrently andassigned to the same assignee, and comprises means for periodicallyfocusing heat radiated from an object to form an image onto an array oftriglycine sulfate crystals (TGS), such as disclosed in copending patentapplication Ser. No. 137,174, filed Apr. 26, 1971, now U.S. Pat. No.3,721,628, and assigned to the same assignee. These crystals have theproperty of converting the radiant heat energy into voltagesrepresentative of the temperature changes of the received images. Meansare provided for transmitting voltages or charges from crystals to anon-conducting plate facing an electron gun that is used to sequentiallyread-out these voltages or charges. The problem solved by applicantsinvention is the rapid. removal of the residual charges that remain onthe non-conducting plate after the electron beam has passed over it.Ordinarily, where a photoconducting matrix is used, there is sufficientdark current flowing during the period of time that the shutter blocksthe reception of radiation from the object. In the use of ferro-electricmaterial, such as Triglycine Sulfate (TGS), there is no dark currentproduced, and other means must be resorted to provide for the removal ofthe residual charges before the shutter allows the next image to bereceived.

It'is peculiar to the pyroelectric effect that the electricalpolarization, p, of a crystal displaying this effect varies withtemperature. The camera pick-up tube described in the precedingparagraph operates in this mode; namely that a thin pyroelectric crystalis exposed to radiations from the object, these radiations cause localchanges in temperatures, and thereby produce changes in polarization ofthe crystal resulting in producing electric charges. If one surface ofthe crystal is rendered electrically conducting and is held at a fixedpotential, then a potential distribution corresponding to the changes inpolarizationis formed on the nonconducting surface. Now once the chargeson the nonconducting side have been neutralized by the electron beam,the target temperatures must be restored to their former values beforethe appearance of new data. This is implemented by the use of a shutterbetween the target and the object to'provide a period of time forrestoring the target to a reference temperature. During this restorationperiod while the shutter blocks the object, charges remaining upon thetarget after the passing of the electron beam have to be removed becausethe target, approximating a perfect insulator provides no leakage pathsfor these residual charges. Although there are known techniques forachieving this result, as will be discussed in the following paragraphs,it is noted that these techniques have inherent disadvantages whichlimit the efficient use of the tube.

One prior art method of electron beam read out is known as"CathodePotential Stabilization (CPS) and is described by P. K. Weimer, S. B.Forgue and R. R. Goodridge in their article entitled The VidiconPhotoconductive Camera Tube, published on pages -73 of vol. 23 ofElectronics magazine in May, 1950, As the electron beam scans across thesurface it functions as a commutator for .the charges deposited thereonand restores the surface potential thereby resulting in a video signalbeing generated in the conducting plate. Since thevideo signal takenfrom the target is capacitively coupled to the scanning surface of thetarget, only electrons are deposited in this mode, and positive ions,attracted to the target for neutralizing are produced from the residualgas in the tube by scanning an electron beam across the target. Thedisadvantage of this method for a pyroelectric vidicon is the inherentlack of electrical conductivity in materials which are suitable forimaging applications.

Another prior art method is known as Anode Potential Stabilization (APS)and is described by J. Dresner in his article entitled TheHigh-Beam-Velocity Vidicon, published between pages 305 and 325 of vol.22 of RCA Review (1961) wherein secondary emission of the anode isproduced so that both positive and negative signals corresponding to.the phase of the shutter result. This mode has the disadvantage that itisnot as uniform as the CPS mode. In a known combination of these twomodes, the target is first scanned while the shutter is between thetarget and the object to render the surface slightly positive and thendischarged, by depositing electrons to attract positive ions producedfrom the residual gas in the tube for neutralizing the target. Then thetarget is exposed to the object and the video signal is read-out fromthe target electrode as described for the CPS mode in the preceedingparagraph.

The process is then repeated for the shutter.

Although the two modes are known, the CPS mode, preferred from a signalto noise performance factor, is not practical because the requiredpedestal current is obtained by using ion currents resulting from a highresidual gas pressure making it difficult to obtain a uniform flux ofions as well as providing a' large number of ions that will bombard theelectron gun cathode, thereby shortening the life of the cathode.

It is an object of the present invention to provide a circuit forremoving residual charges and potentials generated in pyroelectric tubeswhich will result in signal to noise ratios of performance comparable tothose of the CPS mode.

It is a further object of this invention to provide a method that willprovide a much longer life for the cathode than those employing CPSmodes depending upon ion currents.

It is a further object of this invention to provide a method fordirectly reading out the video data from the pedestal current obtainedfrom the non-conducting face of an improved target structure.

SUMMARY OF THE INVENTION Applicants invention employs secondary electronemission, achieved by external circuit means for pulsing the cathode ofthe electron gun from zero potential to a negative potential during thehorizontal flyback the target is greater than unity. Due to thesecondary emission, the successive surface portions of the target areuniformly charged to a positive potential during hoizontal flybackperiods by means of the electron beam, and therefore a uniform pedestalcurrent wll be generated during normal (CPS mode) forward scan.

BRIEF DESCRIPTION OF THE DRAWINGS Applicants invention will be describedin greater detail with reference to the following drawings in which:

direct read-out from the face of the target scanned by the electronbeams of the pyroelectric tube.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1,.apyroelectric thermo imaging television pick-up tube 1, is shown, havingan electron gun 2, with a cathode 3, transparent conducting electrode 8,an array of ferrelectric crystals forming a nonconductive pyroelectrictarget 9, and an optical system including a lens 7 and a window 11.Conventional biasing voltages along with other potentials required fornormal (CPS) mode of the operation of the vidicontype pyroelectric tubeare well known and therefore not shown. External to said pyroelectrictube are shown deflecting means 4, shutter 10 and source 6 of biasingpotential for the electrode 8.

Horizontal and vertical trigger and synchronizing circuits 12 provides ameans for producing the triggers both for negatively pulsing the cathodeand for inhibiting the pulsing of the cathode during horizontal flybackperiods that are within the vertical flyback periods, such asillustrated in FIG. 2. A schematical circuit embodiment for implementingthis is shown in FIG. I, where positive pulses are supplied by thehorizontal and vertical triggers and synchronizing circuits 12 to thepulse amplitude circuit 13 and the cathode pulse amplitude circuit 14.Negative pulses from the pulse amplitude circuit 13 are then supplied tothe control grid of the pyroelectric tube through pulse amplifier 15.The negative pulses from pulse amplifier are delayed to occur at thesame time as inhibiting pulses from the horizontal and vertical triggerand synchronizing circuits 12. Cathode pulse amplitude circuit 14supplys pulse amplifier l6 and this in turn is coupled to cathode 3 ofthe pyroelectric tube. Although the video output is shown in FIG. 1 tobe taken off electrode 8, it is noted that an improved target design tobe described later provides another method of obtaining the pedestalcurrent.

FIG. 3 shows schematic drawing for the anode pulse amplitude and anodepulse amplifier circuits I3 and 15 and the cathode pulse amplitude andthe cathode pulse amplifier circuits l4 and 16. The input to resistor R1is a positive pulse from the horizontal and vertical trigger andsynchronizing circuits l2. Resistor R1 provides two paths; one of whichis coupled to capacitor C1 and then through the parallel circuit ofcapacitor C2 and resistor R3 to the base of npn transistor 01 operatingto invert as well as amplify the input positive waveform to a negativepulse whose amplitude is determined by the adjustable voltage dividercomprising resistors R4, R5 and R6. NPN transistors Q2 coupled to theoutput of the voltage divider operates as an emitter follower to matchthe output of the cathode pulse amplifier 16 to the input cathodecircuit of the pyroelectric tube. The other path for the positive pulseof resistor R1 is coupled through capacitor C3 and the parallel circuitcomprising capacitor C4 and resistor R9 to the base of N PN transistorQ3 operating to invert as well as amplify the input waveform to anegative pulse whose amplitude is determined by the adjustable voltagedivider comprising resistors R10 and 11. This inverted pulse having apredetermined amplitude and delayed a predetermined interval time isconnected to the base NPN transistor Q4 operating as an emitter followerwhich thereby couples this inverted pulse through capacitor C5 and diodeD1 to the output of horizontal and vertical trigger and synchronizingcircuits 12 as well as to the control grid of the electron gun of thepyroelectric tube. The video output of the pyroelectric tube is eithertaken off conducting electrode 8 which is coupled to capacitor C6 or asdescribed later in the following paragraphs. Conducting electrode 8 isbiased by battery, through resistor R14.

Operating ofthe above described circuit is as follows: Non-coincidinghorizontal and vertical triggers are applied to horizontal and verticaltrigger and synchronizing circuits 12 to produce negative pulses at thecathode of the pyroelectric tube during the horizontal flyback period,provided the horizontal fiyback period does not coincide with thevertical flyback period. If the vertical and horizontal flyback timescoincide, an inhibiting pulse is produced by the circuits 12 at theanode of the pyroelectric tube, negating the effect of the negativecathode pulse. Pulsing the cathode of the pyroelectric tube results insecondary emission in the tube which produces a positive potential onthe nonconducting target and this will produce a uniform pedestalcurrent during the normal forward scan.

FIG. 4 shows a target design for improving gain of a pyroelectric tubecomprising pyroelectric substrate 17 which has a transparent conductivefilm 18 covering one surface, and a number of parallel conducting strips19, spaced a distance apart from each other on the opposite, surface ofthe substrate. The spacing between the conducting strips is very close;being determined by the resolution of the image desired. The surface ofthe substrate having the conducting strips would be positioned in thepyroelectric tube shown in FIG. 1. The operation of this target issimilar to that of a coplanar triode; the conducting strips functions asthe plate and the pyroelectric material between the conducting stripsfunctioning as the grid.

FIG. 5 shows an improved target design of FIG. 4 having a resistive film20 covering the conducting strip and surface of the pyroelectricsubstrate to provide leakage paths for the residual charges and thepotentials generated during the restoration periods while the tube isbeing operated. A pedestal current is obtained by means of these leakagepaths. Since such leakage current represents the temperature-voltagecharacteristics of the received image, this current can be readout forprocessing by conventional means.

I claim: 1. A circuit arrangement for producing a substantially uniformpedestal current in a vidicon-type thersaid one side of the targetrelative to the cathode to a surface potential required for cathodepotential stabilization during the forward scan of said beam, and meansfor negatively pulsing said cathode relative to the target duringhorizontal flyback periods not coinciding with vertical flyback periods,the amplitude of said negative pulses having a value at which thesecondary emission ratio of said target for electrons impacting uponsaid one side of the target is greater than unity.

2. A circuit arrangement according to claim 1, wherein said negativelypulsing means include means for producing timing trigger pulsescoinciding with flyback periods of horizontal and vertical deflectionsignals, respectively, and means for deriving negative pulses from thetrigger pulses coinciding with flyback periods of the horizontaldeflection signals.

3. A circuit arrangement according to claim 2 further including meansfor inhibiting the negative pulses applied to said cathode when saidhorizontal trigger pulses coincide with said vertical trigger pulses.

4, A circuit arrangement as claimed in claim 1,

wherein said means for negatively pulsing the cathode of saidpyroelectric tube comprises both amplitude and phase control circuits.

5. A circuit arrangement as claimed in claim 1, wherein said targetscanned by the electron beam within a pyroelectric therrno imaging tubecomprises a non-conducting substrate, a transparent conducting filmcovering the surface of said substrate, opposite to the surface facingthe electron beam, a plurality of parallel conducting strips spacedapart from each other abutting the surface of said substrate, facing theelectron beam the spacing between said conducting strips beingdetermined by the resolution of the radiation of the received image ofthe pyroelectric tube, and means for producing D-C bias between theconducting strips and said conducting film.

6. A circuit arrangement as claimed in claim 5, further comprising aresistive film covering the surface facing the electron beam andabutting conducting strips to provide a leakage path for the target andmeans for obtaining said pedestal current from said leakage path.

7. A circuit arrangement as claimed in claim 6, further comprising meanscoupled to said resistive film to read-out the data representing thetemperature changes of the received images.

8. A circuitarrangement for producing a substantially uniform pedestalcurrent at the side of a nonconductive pyroelectric target scanned by anelectron beam within a vidicon-type thermo-imaging tube biased for acathode potential stabilization, comprising means for negatively pulsingthecathode ofsaid tube during horizontal flyback periods of said beam,the amplitude of the negative pulses having a value at which thesecondary emission ratio of said target for electrons impacting uponsaid one side of said target is greater than unity.

I UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3.8124396 I Dated I May 21, 1974 Inv entorM/ MERTON H. CRGNELL It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

Column 1, line 13, delete (154-10-911) filed concurrently and I insertthe following: -Serial No.

322,633, filed January 11, 1973, now

abandoned and Serial No. 45l,882, filed March 18, 1974 which is acontinuation of said application-.

Signed and sealed this 5th day of November 1974.

(SEAL) attest: I I I I McCOY M. GIBSON JR. c. MARSHALL DANN AttestingOfficer Commissioner of Patents UNITED STATES PATENT OFFICE CERTIFICATEOF CORRECTION Patent No. 3 12 39 at d Ma 21, 1974 lnventorl MER'ION H.cRowELL It is certified that error appears in the above-identifiedpatent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 13, delete (154-10-911) filed concurrently/1'1 and insertthe following: Serial No.

322,633, filed January 11, 1973, now

abandoned and Serial No. 45l,882, filed March 18, 1974 which is acontinuation of said application.-

Signed-and sealed this 5th day of November 1974.

(SEAL) attest:

McCOY M. GIBSON JR. c. MARSHALL DANN Attesting Officer Commissioner ofPatents

1. A circuit arrangement for producing a substantially uniform pedestalcurrent in a vidicon-type thermo-imaging tube having a target ofnon-conductive pyroelectric material, an electron gun with a cathodenormally at substantially zero potential relative to the target and witha control electrode for directing an electron beam against one side ofsaid target, and means for horizontally and vertically deflecting saidelectron beam according to a predetermined scanning pattern, saidcircuit arrangement comprising means for biasing said one side of thetarget relative to the cathode to a surface potential required forcathode potential stabilization during the forward scan of said beam,and means for negatively pulsing said cathode relative to the targetduring horizontal flyback periods not coinciding with vertical flybackperiods, the amplitude of said negative pulses having a value at whichthe secondary emission ratio of said target for electrons impacting uponsaid one side of the target is greater than unity.
 2. A circuitarrangement according to claim 1, wherein said negAtively pulsing meansinclude means for producing timing trigger pulses coinciding withflyback periods of horizontal and vertical deflection signals,respectively, and means for deriving negative pulses from the triggerpulses coinciding with flyback periods of the horizontal deflectionsignals.
 3. A circuit arrangement according to claim 2 further includingmeans for inhibiting the negative pulses applied to said cathode whensaid horizontal trigger pulses coincide with said vertical triggerpulses.
 4. A circuit arrangement as claimed in claim 1, wherein saidmeans for negatively pulsing the cathode of said pyroelectric tubecomprises both amplitude and phase control circuits.
 5. A circuitarrangement as claimed in claim 1, wherein said target scanned by theelectron beam within a pyroelectric thermo imaging tube comprises anon-conducting substrate, a transparent conducting film covering thesurface of said substrate, opposite to the surface facing the electronbeam, a plurality of parallel conducting strips spaced apart from eachother abutting the surface of said substrate, facing the electron beamthe spacing between said conducting strips being determined by theresolution of the radiation of the received image of the pyroelectrictube, and means for producing D-C bias between the conducting strips andsaid conducting film.
 6. A circuit arrangement as claimed in claim 5,further comprising a resistive film covering the surface facing theelectron beam and abutting conducting strips to provide a leakage pathfor the target and means for obtaining said pedestal current from saidleakage path.
 7. A circuit arrangement as claimed in claim 6, furthercomprising means coupled to said resistive film to read-out the datarepresenting the temperature changes of the received images.
 8. Acircuit arrangement for producing a substantially uniform pedestalcurrent at the side of a non-conductive pyroelectric target scanned byan electron beam within a vidicon-type thermo-imaging tube biased for acathode potential stabilization, comprising means for negatively pulsingthe cathode of said tube during horizontal flyback periods of said beam,the amplitude of the negative pulses having a value at which thesecondary emission ratio of said target for electrons impacting uponsaid one side of said target is greater than unity.